I. Mechanisms of Platelet Activation Elevation of cytosolic calcium is thought to play a major role in platelet activation by physiologic agonists or by extended storage. The rise in cytosolic calcium induces a variety of biochemical responses including tyrosine phosphorylation ad dephosphorylation of proteins. A tyrosine kinase inhibitor, erbstatin, was found to selectively potentiate fibrinogen binding-mediated tyrosine phosphorylation of a single 95 kDa protein following platelet activation. The mechanism of selective tyrosine phosphorylation of this protein appears to be a result of the disruption of the translocation of phosphatases to the cytoskeleton during platelet activation. Isolation and sequencing of the 95 kDa protein is being pursued. Current platelet storage conditions, which utilize platelet-rich plasma at room temperature, are associated with a progressive platelet activation and are a good media for bacterial growth if contamination occur during collection. An ideal synthetic storage media is one that would prevent bacterial growth and extend the platelet storage period beyond five days. In the search for this media, it is necessary to evaluate platelets stored in the new medium against conventionally stored platelets. We have found that the selection of appropriate resuspension media after storage significantly affects the performance of the platelets in in vitro tests. The resuspension medium must be the same for both test and control platelets so that a fair comparison of platelets stored under novel conditions may be performed. II. Prion Proteins and Platelets The prion protein (PrPC) is expressed on many cell types and tissues and is thought to be the infectious agent in the fatal neurodegenerative diseases called transmissible spongiform encephalopathies (TSE). We have investigated the role of PrPC in platelet physiology. The surface expression of PrPC, as detected by flow cytometry with a PrPC-specific monoclonal antibody, increased more than twofold after platelet activation. The GPI-linked protein was resistant to removal by phospholipase C, a characteristic found with the pathologic form of PrPC, but not the physiologic form of PrPC. The platelet PrPC appears to be localized in a granule and is transferred to the outer plasma membrane upon activation. The function of PrPC may be in mediating heterozygous platelet-cell interaction and/or adhesion.